X-ray windows are designed to allow high transmission of x-rays, even low-energy x-rays. For some applications, it can be important for x-ray windows to block visible and infrared light transmission, in order to avoid creating undesirable noise in sensitive instruments.
It can be important for x-ray windows to be strong, because the x-ray window may need to withstand a differential pressure of about 1 atm. X-ray windows need sufficient thickness for strength, but not a thickness that will cause excessive attenuation of x-rays. A vacuum on one side and air on an opposite side can cause the x-ray window to bow or deflect, damaging the x-ray window, and also possibly causing a short circuit by creating an unintended electrical-current path. Thus, it can be important to minimize deflection.
It can be important for x-ray windows to be substantially impervious to gases. It can be important for the x-ray window to form a hermetic seal.